1. Field of the Invention
This application relates to an air flow gauge, more particularly to an air flow gauge intended for use in conjunction with a paint spraying gun atomization circuit.
2. Description of Related Art
Many factories use compressed air as a source of power for operating various types of production equipment. xe2x80x9cCompressed air,xe2x80x9d which is sometimes referred to as xe2x80x9cpressurized airxe2x80x9d or referred in spray paint operations as xe2x80x9catomization air,xe2x80x9d is defined as free air that has been compressed into a volume that is smaller than the volume the air normally occupies at normal atmospheric pressure. Controlled expansion of the compressed air can be used as a source of power to operate a wide range of pneumatically powered tools. Compressed air is typically supplied from onsite or nearby compressors and piped through a distribution system to a downstream point of use. Paint spraying operations for painting various types of manufactured products, such as automobiles, is one typical use of compressed air.
In a spray paint operation, a paint fluid, which can be in the form of either a liquid or a fine powder, is mixed together with compressed air in a spray gun nozzle in order to atomize the paint into extremely fine particles and to transfer the paint particles onto the surface of the item being painted. One commonly used spray paint gun, referred to in the industry as a high volume low pressure (HVLP) spray gun, generates high volumes of low pressure air that propel the paint particles from the nozzle of the gun toward the surface of the article being painted. Other uses of compressed air include pneumatically powered drills, wrenches and other types of machine tools. The optimum operation of such tools depends upon providing a specified flow rate of compressed air to the tool.
Industrial compressed air systems are commonly controlled by pressure regulation, meaning by regulating the nominal air pressure at a certain point in the system. A pressure regulator might be placed, for example, at or near the compressor, at one or more points on the distribution line, or on a hose used to supply air to the tool. The major disadvantage with this method is that measuring air pressure at only one particular point within the system is not necessarily a good indicator of the volume of air flowing though that particular point in the system. Air pressure drops as it flows through the system, and the amount that the air pressure drops from one point to the next varies greatly depending on the specific system installation and also on varying conditions of usage occurring during the course of the day. For example, in many cases a compressed air system supplies not only spray guns but also other devices used in a paint shop such as sanders, polishers, screw drivers, drills and so forth.
For paint spray operations in particular, one commonly used method for determining whether a sufficient amount of compressed air is being delivered to the spray gun is to place a pressure gage on the cap of the spray gun immediately after the painter has set the spray gun for proper atomization of the paint, but before he actually begins painting. Many operators, however, find this extra step to be a great inconvenience as it interrupts their painting operations. Therefore, this procedure is often disregarded. Another method of checking whether a sufficient amount of compressed air is being delivered to the spray gun is to attach an air gauge to the handle of the gun. However, attaching a pressure gauge to the gun naturally increases its weight. Over a period of time, muscle fatigue sets in, thereby causing the operator to use unnatural arm and wrist actions which, in turn, cause over spray or under spray conditions and other flaws in the paint job.
More importantly, regulating the nominal pressure at any one point in the system does not necessarily mean that the proper amount of air, or even any air is flowing at another point downstream. For example, there may be blockage in the spray nozzle of a paint gun, or a break in the line or some other problem in the system.
Another problem with the traditional method of using a single pressure gauge to monitor air flow in a high pressure line is that the particular gauge being used must be able to withstand the high pressure of the spray system. In paint spray systems, the liquid paint is atomized under high pressure, typically in the range of about 10 p.s.i. for a HVLD spray gun, 25 to 60 p.s.i. for a dynamic air spray and 100-125 p.s.i. for a static air spray. Thus, pressure gauges used on such systems are therefore typically made from very heavy and bulky components and consequently lack the resolution necessary to accurately measure the difference between, for example, 8 and 9 p.s.i. Traditional monitors also lack repeatability as mass and hysteresis of the moving components of the gauge effect the movement of gauge needle. Thus, accurately regulating the pressure and flow rate of air in a spray system is extremely difficult.
It is advantageous to monitor airflow through a spray gun to assure proper performance. Restrictions in the air delivery hose, gun body, and spray gun cap can greatly affect the airflow through them. Varying conditions of hose length, delivery pressure, and supply air temperature also affect pressure and flow rates. For monitoring the air flow it is desirable to use an inline circuit monitor which can be connected to the components to be monitored rather than to disassemble a spray gun system and take the component to a test bench to test the flow rate. An inline flow gauge is especially important when the spray gun is part of an automated machine and is not designed to be removed easily from the control system.
Various types of gauges such as floating ball, turbine, thermal, ultra-sonic, and differential pressure gauges have been used to measure the flow rate of air in high pressure air systems. Such devices are commonly calibrated so that their scales read in terms of cubic feet or liters per minute. They must be carefully made so as to be accurate, yet at the same time, they must be able to withstand the high pressures and also sudden pressure changes or surges that commonly occur in industrial paint spray air systems. As a result, these flow meters typically constitute the most expensive single element in an inline monitor.
Virtually all known monitoring devices for high pressure systems have a number of drawbacks. As mentioned, heavy gauges can withstand sudden pressure changes, yet they cannot accurately measure small pressure differences. Additionally, the narrow resolution of heavy gauges makes them less effective when the equipment is working at low pressures. Light gauges can accurately measure small pressure differences, but they cannot withstand the high pressures and sudden pressure changes in typical paint spray lines. Finally, known differential pressure measuring devices such as floating ball gauges and other devices mentioned above are too expensive to use on a plant-wide basis for many users.
Accordingly, there is a need for an improved air flow rate monitoring device or gauge that can accurately measure the air flow of sprays within high-pressure paint spray systems which can also be manufactured at a considerably lower cost than other flow gauges using differential measuring methods.
An improved flow rate meter than can accurately measure and regulate flow of sprays in high-pressure paint spray systems is disclosed. The invention, which is defined by the claims set out at the end of this disclosure, is especially designed and adapted to address several of the drawbacks noted above with respect to the use of conventional, heavy-duty high pressure gauges. Specifically, the air flow rate meter disclosed herein provides an accurate measurement of air flow rates operating at high pressure values and can also withstand sudden pressure drops and surges.
The air flow rate meter disclosed herein comprises a means for providing a low pressure flow meter which can be used in conjunction with gas lines of much higher pressure than the meter or gauge is rated for. The means comprises a meter or gauge mounted within a sealed housing that is connected to the gas line. By mounting the low pressure gauge within the housing, and having the pressure within the gauge and housing equalized with the air pressure in the line, the gauge can be used to detect any pressure drops or surges within the gas line. The gauge is connected to a restricted orifice to measure the pressure drops or surges in the gas line, which directly correspond to the rate (i.e., volume) of air flowing through the line.
More specifically, the air flow rate meter of the present invention comprises a low-pressure meter, (e.g., 0 to 5 p.s.i.), encapsulated within a high-pressure vessel, with both the low pressure meter and the high pressure vessel pneumatically connected to the high pressure air line. Although the system pressure may be extremely high (e.g., 100 to 125 p.s.i. or more), the gauge of the present invention is designed to measure a relatively small pressure drop through a known restriction at a specific point in the gauge. Due to the relationship between a pressure drop between opposed ends of a restriction having known dimensions and the flow rate through the restriction, the flow rate can be measured by calculating the pressure drop across the known restriction.
The durability of the proposed invention is dependent upon equalizing the pressure within the pressure vessel with the outside system pressure. Once the entire system is up and running and fully pressurized, the air pressure in the low pressure gauge and the pressure vessel naturally becomes equalized to the system pressure. However, during startup and shut down, the difference in pressure between the airline and the vessel may exceed the capacity of low-pressure meter and, unless the meter is effectively isolated, damage the internal parts of the meter. Therefore, the vessel further comprises a pressure regulator for regulating the input pressure during startup, and a check value for equalizing the pressure across the gauge during inlet pressurization.